Lipid rafts are dynamic, heterogeneous micro domains forming specialized regions in cell membranes, and are enriched in cholesterol and glycolipids, most evidently sphingolipids such as GMI (Ross & Pawlina, 2006; Pollard & Eamshaw, 2002). One definition of them in vitro is as remaining insoluble after extraction with the detergent Triton X-100, used diluted and in the cold. Such a detergent resistant membrane fraction is recovered as a low-density band isolated by flotation gradients. The prevalence of lipid rafts is reduced by depletion or disorganization of cholesterol in cell membranes. Lipid rafts are in close relation to and connected to the cytoskeleton, and involved in cell polarization. They constitute specialized micro domains, prevalent in membranes in cells and tissues at all ages e.g. in embryos, fetuses as well as in young, adult and old individuals.
Proteins of key importance for cohesion and signal transduction are enriched in the lipid rafts, such as receptors, cell attachment proteins, ion transporters, and ion channel complexes including e.g. aquaporines, as well as chemokine receptors, neurotransmitter receptors, hormone receptors and growth factor receptors. These proteins and protein complexes are interacting with the intracellular G protein systems, which transfer the message received by e.g. the receptors to the cell's cytoplasm and nucleus (Dermine et al., 2001; Ross & Pawlina, 2006; Pollard & Earnshaw, 2002; Helms & Zurzolo, 2004; Chini & Parenti, 2004; Head et al., 2006; Mahmutefendic et al., 2007). The distribution and concentrations of the key ion in monitoring cellular activity, Ca2+, has been demonstrated to be closely related to lipid rafts and to caveolae. Additional proteins, such as connexins, CD38, CD19, Thy-1 and CD59, are anchored to lipid rafts, commonly via glycosylphosphatidylinositol (GPI) anchored proteins and receptors, which enables them to interact with cell functions. Sphingolipids, as illustrated by the ganglioside GMI, the target for cholera toxin, are enriched in and characterizing lipid rafts. In addition, neurotransmitter receptors and other constituents of synapses and neuronal processes as well as growth factor receptors are among the wide range of proteins prevalent in the lipid rafts e.g. in such highly specialized cells as neurons. Calcium ion channels and transporters, of key importance in regulating cell functions and interactions, are to a large extent confined to lipid rafts (A Spat, 2007). This may be illustrated by the observation that disruption of lipid rafts hampers or even prevents a cell's ability to propagate traveling Ca2+ waves in cells. Further, calcium ion bursts significantly influence key functions in normal cells and in pathologically altered cells, e.g. cell division, cell survival and cell death.
Further, lipid rafts are considered to play a key role in intracellular protein trafficking, receptor and lipid dynamics. Messages and actions by all these receiving and conveying proteins are transmitted to the interior of cells via G-linked protein systems, via enzyme systems or with the aid of the cytoskeleton (Triantafilou & Triantafilou, 2004). The physical state of lipid rafts is e.g. known to be conducive to concentrate and constrain the mobility of multiple proteins to facilitate the dynamic assembly of competent signaling complexes.
What is more, lipid rafts are considered to have the capacity to regulate activation, signaling and rearrangement of the cytoskeleton, which renders them critically important to the mechanisms governing cellular locomotion including directional migration, as well as for maintaining the shape and size of cells and associated transport. The cytoskeleton is further of key importance for intracellular trafficking of cell constituents and for sensing dynamic and static load on cells.
Lipid rafts are thus dynamic structures, usually with a diameter in the order of 5-50 nm, with a considerable range of variation. There are a multitude of approaches to demonstrate the presence of lipid rafts, e.g. by immunohistochemical and immunochemical demonstration of GMi which has a very high affinity to cholera toxin. Another way to demonstrate their presence and position is to isolate cell membranes after having disrupted the cells and then isolated a detergent resistant fraction at a defined temperature, as described in commonly available cell biology textbooks.
Flotillin, caveolin and reggie may be used to immunohistochemically identify lipid rafts. Of those proteins, flotillin may further associate with lipid droplets. Atomic force microscopy and related approaches add to the techniques enabling the visualization of lipid rafts. Exposure of cells to cyclodextrine and variants thereof, causing a depletion of cholesterol from the membranes, constitutes an alternative way of demonstrating the prevalence of lipid rafts.
Caveolae constitute a specialized type of lipid raft. They are dynamic structures characterized by focal enrichment in membranes of cholesterol and sphingolipids, transducing signals between the environment and the interior of cells as well as connecting to the cytoskeleton. In contrast to other lipid rafts, caveolae are larger and usually appearing as flask-shaped pits or invaginations in the cell membranes and as vesicles (Kurzchalia & Parton, 1999). Their size is commonly in the order of 0.1 pm, but with considerable variations. Caveolae are prevalent in e.g. cardiac and smooth muscle cells, endothelial cells, macrophages, and adipocytes, i.e. in virtually all-mammalian cells although in a highly varying frequency.
Lipid rafts and caveolae have been disclosed to harbor and influence NO (nitrogen oxide) generating system. In addition to conveying signals to and from a cell, caveolae are involved in the trafficking of fluid and various compounds to and from a cell, endocytosis and the regulation, trafficking, efflux and maintenance of fatty acids and cholesterol in cells and their environment (Pohl et al., 2004; Rajendran et al., 2007). Caveolae constituents and lipid rafts are further involved in the processing of 13 amyloid precursor protein ((3APP) and amyloid p (A(3), proteins related to preferentially Alzheimer's diseases but also to other neurodegenerative disorders and neurotrauma (Graham & Lantos, 2002).
Tumor cells are known to have lipid rafts and as well caveolae. Thus, it is e.g. possible to impair the growth and migration of tumor cells by disrupting or to a variable extent disintegrating these structures (Marquez, D C et al., 2006; Freeman et al., 2007).
The importance of lipid rafts and caveolae has been further elucidated in genetically modified animals. Knockout mice deficient in caveolin develop dilated cardiomyopathy and pulmonary hypertension (Mathew et al., 2004). Further, lipid rafts and caveolae are related to insulin stimulated transporter systems such as steroid hormone conveying systems.
It is concluded that lipid rafts and caveolae, constituting highly dynamic structures, closely related but in some aspects also diverging, are prevalent in mammalian cells and exert a multitude of important functions. Approaches available at present enable the disruption or depletion of lipid rafts and caveolae, but there is no known way to restore and/or normalize the structure, distribution, frequency and/or function of lipid rafts and signaling and mass transfer proteins, organized in the lipid raft.
The antisecretory protein is a 41 kDa protein that originally was described to provide protection against diarrhoeal diseases and intestinal inflammation (for a review, see Lange and Lonnroth, 2001). The antisecretory protein has been sequenced and its cDNA cloned. The equivalent activity seems to be mainly exerted by a peptide located between the positions 35 and 50 of the antisecretory protein sequence. Immunochemical and immunohistochemical investigations have revealed that the antisecretory protein is present in and may also be synthesized by most tissues and organs in a body. Synthetic peptides, comprising the antidiarrhoeic sequence, have been characterized (WO 97/08202; WO 05/030246). Antisecretory factors have previously been disclosed to normalize pathological fluid transport and/or inflammatory reactions, such as in the intestine and the choroid plexus in the central nervous system after challenge with the cholera toxin (WO 97/08202). Use of natural antisecretory factors to food and feed was therefore suggested to be useful for the treatment of edema, diarrhea, dehydration and inflammation in WO 97/08202. WO 98/21978 discloses the use of products having enzymatic activity for the production of a food that induces the formation of antisecretory proteins. WO 00/038535 further discloses the food products enriched in antisecretory proteins as such.
Antisecretory protein and fragments thereof have also been shown to improve the repair of nervous tissue, and the proliferation, apoptosis, differentiation, and/or migration of stem and progenitor cells and cells derived thereof in the treatment of conditions associated with loss and/or gain of cells (WO 05/030246).
Antisecretory factors (AF), specifically proteins and peptides, as described in detail in WO 97/08202, are effective in abolishing hypersecretory conditions and diseases in the intestine, such as diarrhea. Other examples related to effects of AF in relation to hypersecretory conditions are e.g. inflammatory bowel diseases, brain edema, glaucoma, elevated intracranial pressure, Morbus Méniére, and mastitis. AF has as well been considered for the treatment of glaucoma (WO 97/08202).
It has recently been recognized that the structure, prevalence, distribution and function of lipid rafts, which have been altered due to abnormal cell function, excessive or abnormal load, infection, or by a toxic compound or a drug may be monitored and even normalized with the aid of certain specific proteins and related compounds.
Astonishingly enough, the inventors have now been able to prove that the protein Antisecretory Factor (AF) and peptides derived thereof, e.g. AF-16 and AF-8, can affect the structure, prevalence, distribution and/or function of lipid rafts, receptors and/or caveolae, which have been altered due to abnormal cell function, excessive or abnormal load, infection, or by a toxic compound and/or a drug in cells, tissues and/or organs, in such a way that it is for the first time possible to monitor, control and/or even normalize the functions confined to or related to lipid rafts and/or caveolae.